1. Field of the Invention
The present invention relates to an electroconductive rubber roller such as a charging roller, a developing roller or a transfer roller, adapted for use in an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus, as represented by a copying apparatus, a printer or a facsimile apparatus.
2. Related Background Art
An image forming apparatus of electrophotographic process, such as a copying apparatus or a printer, has employed a process of depositing a toner on an electrostatic latent image and transferring such toner image onto a recording medium such as a transfer paper, thereby obtaining a print. More specifically, a photosensitive member is charged uniformly on its surface and is then exposed to an image projected by an optical system to dissipate the electrostatic charge in an exposed portion to form a latent image. Then, toner is deposited to form a toner image (development), which is then transferred to the recording medium such as a transfer paper to obtain a print.
For uniformly charging the surface of the photosensitive member, there is known a contact charging method of bringing a charging member, to which a voltage (for example 1 to 2 kV) is applied, into contact with the photosensitive member by means of a predetermined pressing force thereby charging the photosensitive member to a predetermined potential. In such method, a charging roller is commonly employed as it can easily realize a uniform contact with the photosensitive member, which is a key factor for achieving a uniform charging in the contact charging method and which can be effected by means of two rotating cylindrical members, in comparison with other contact charging methods such as a brush charging method or a blade charging method.
Such charging roller, for attaining a uniform and satisfactory chargeability in contact with the photosensitive member, is required to have a large and uniform contact area (nip width) in the contact between the charging roller and the photosensitive member. For this purpose, the charging roller is also required to have an appropriate hardness (low hardness). Also the charging roller, being deformed in contact, is required to have a sufficient recovering property against compressive force. On the other hand, in order to apply a necessary bias voltage to the charging roller, the charging roller is required to have a low volume resistivity and has to be regulated so as to be of a desired current value. Also in case the charging roller is electrically not uniform, the charging density on the photosensitive member becomes uneven reflecting such electrical non-uniformity. Therefore, the charging roller is required to have a predetermined resistivity and to be electrically uniform. Thus, various physical properties are required for the charging roller, which comes into direct contact with the photosensitive member.
Furthermore, in the rubber roller employed as the charging roller, its volume resistivity has to be within a predetermined semiconductive range of 1×105 to 1×1010 Ω·cm. For realizing such desired electroconductivity, the charging member has been produced by a method of adding and dispersing an electroconductive filler such as carbon black, or a method of selecting rubber having electroconductivity in itself. In the method of adding and dispersing electroconductive filler, the electrical characteristics are influenced even by a small fluctuation in the amount of the filler or a dispersion state and an orientation thereof. Therefore fluctuation in the performance tends to be generated from batch to batch in kneading and from roller to roller even in a same batch. Also the roller obtained in such method shows a significant dependence on the applied voltage, and a stable volume resistivity is difficult to attain. On the other hand, in the method utilizing electroconductive rubber material, such fluctuation is scarcely encountered and the desired electroconductivity can be easily regulated and obtained in stable manner. For this reason, the rollers utilizing electroconductive rubber are recently increasing along with the higher performance required for products.
Such electroconductive rubber can generally be acrylonitrile-butadiene rubber, epichlorohydrin-based rubber or acrylic rubber. Among these, epichlorohydrin-based rubber is known to be a polymer of low electrical resistance.
As the epichlorohydrin-based rubber an epichlorohydrin homopolymer is known. Also there are known an epichlorohydrin-ethylene oxide copolymer, an epichlorohydrin-allyl glydicyl ether copolymer, and an epichlorohydrin-ethylene oxide-allyl glycidyl ether copolymer. Such epichlorohydrin-based rubber has a property that the resistance can be controlled by changing the copolymerizing proportion of ethylene oxide constituting such rubber, and it is already known that the volume resistivity becomes lower as the copolymerizing proportion becomes higher.
However, an epichlorohydrin-based rubber with a high copolymerizing proportion of ethylene oxide, for example employed for preparing an electroconductive rubber roller of a low volume resistivity, is very difficult to provide electroconductive rubber rollers of desired performance in stable manner. This is because in some cases such epichlorohydrin-based rubber, when employed in the preparation of rollers, results in a significant fluctuation in the performance of the rollers from lot to lot, also does not achieve a sufficient reduction in the volume resistivity of the rollers, also results in an unstable rubber hardness of the rollers after vulcanization and may lead to rollers of an abnormally low current value. With a low current value, the roller is difficult to attain a required charging ability, and, when incorporated in an electrophotographic image forming apparatus, is unable to provide a satisfactory image. Also with a high hardness, it is unable to secure a sufficient nip width when in contact with the photosensitive member and it may even cause a damage thereon. These are fatal drawbacks in exhibiting the properties required for the electroconductive rubber roller.
For solving these drawbacks, Japanese Patent Application Laid-open No. 2000-063656 discloses a vulcanizable material including an ether-type copolymer, of which vulcanized substance shows, at 23° C. and 50% RH, a volume resistivity of 1×105 to 2×107 Ω·cm and an environmental dependence of 2.5 or less, for use as a semiconductive material for a low-resistance electroconductive rubber roller.
However, the electroconductive rubber roller prepared with such material, though certainly showing an electroconductivity within the above-indicated range of the resistance value, shows a significant fluctuation in the electroconductivity from lot to lot of material, and is poorly practical as the electroconductive rubber roller for the image forming apparatus.
Also for obtaining an electroconductive rubber roller of an appropriate hardness, a plasticizer is often added to the epichlorohydrin-based rubber. However, when the plasticizer is one ordinarily employed in the epichlorohydrin-based rubber, it may exude out from the electroconductive roller in a prolonged contact with the photosensitive member to cause a local deterioration thereof (stain on photosensitive member), thereby detrimentally affecting the reproduction images. Therefore, the usable plasticizer has been limited in terms of the type and the blending amount thereof.